On the market there is today a number of milk products with different fat content packed in packages. In Sweden there is for example minimum milk 0.1% fat, light milk 0.5% fat, "middle" milk 1.5% fat and "standard" milk 3.0% fat. Abroad there are products with other fat contents. The fat content given on the packages must be kept and production of milk with a certain, guaranteed fat content, usually called standardization of milk, is therefore an important process step in a dairy.
The standardization process has hitherto been carried out according to two different methods, batch-wise in tanks or by direct in-line standardization. The first step according to both these methods is to separate whole milk into cream and skimmilk. The term "whole" milk or raw milk is used for milk as it is delivered to the dairies with a fat content around 4%.
When carrying through the standardization in batches two methods are used, namely pre-standardization and post-standardization. The pre-standardization means that the milk is standardized prior to the pasteurization. When the milk shall be standardized to a fat content higher than the fat content in the raw milk, cream is mixed with the milk in proportions which shall give the desired fat content. If the standardization shall take place to lower fat content, the raw milk is diluted with skimmilk. The standardized milk is pasteurized after analysis of the fat content and a possible correction of the same.
Post-standardization means that pasteurized milk is mixed with cream or skimmilk depending on if the fat. content shall be adjusted upwards or downwards in the same way as practiced for the pre-standardization. Some risks for re-infection are present since post-standardization means mixing of already pasteurized products. Both methods demand large tanks and analysis and a possible correction of the fat content is labour-intensive.
The second method, direct-standardization, has therefore been an attractive alternative for many years. According to this method the fat content is adjusted to the required level by remixing a certain amount of the cream obtained from the separator with skimmilk also obtained from the centrifugal separator. This remixing takes place in the skimmilk pipeline connected to the skimmilk outlet from the centrifugal separator.
The pasteurization usually takes place in connection with the standardization. Whole milk is preheated to a temperature suitable for the separation of the cream from the skimmilk. The standardized milk that is obtained after the remixing of suitable amounts of cream and skimmilk after passage of the centrifugal separator is then heated and pasteurized.
Many methods which make an automatic direct-standardization of the milk possible has been developed and are today operating in a number of dairies.
The methods described above are used for manufacture of milk with a preselected fat content. A desired amount of milk is produced. If milk with another fat content should be produced, the system must be adjusted to new preset values and volumes.
The different milk products with a varying fat content mentioned above may be produced by in-line standardization. The packaging does not take place immediately after the standardization but the milk products are intermediately store in large tanks in the dairy. Usually, different kinds of consumer milk are packaged firstly when the distributors working on the market have delivered their orders for a suitable amount of packages of consumer milk with differing fat content.